The present invention relates to an apparatus for cross-flow filtration.
Extremely finely pored filter membranes used during ultra-filtration and reverse osmosis in industry require considerably high pressures and large surface areas. For manufacturing and economic considerations it is therefore desirable to arrange as much filter area as compactly as possible in a pressure chamber. To avoid clogging of the pores and salt polarisation, respectively, the liquid to be filtered is carried at a considerable flow velocity parallel to the filter surface, i.e. in a turbulent state, so that alluvial particles of colloidal to molecular size are constantly removed from the membrane or diaphragms. With the so called "cross-flow" filtration, the liquid is generally repeatedly carried past the filter diaphragms until the concentration of dissolved particles increases to a predetermined limit.
Generally, tubular filter elements are used for cross-flow filtration and have the advantage of a low pressure drop but have a low filter area per unit. Moreover, the manufacture of tubular membranes having a uniform pore diameter requires a considerable wall thickness of the filter elements and is limited with respect to the choice of materials.
The use of disc-shaped filter elements has therefore been proposed for cross-flow filtration. In U.S. Pat. No. 3,456,805 and No. 3,398,833, the filter elements are arranged in the form of a labyrinth seal, such that the medium to be filtered flows around each individual filter element and thereby experiences a reversal of about 180.degree.. Due to this labyrinth construction, however, the pressure drop is quite high and the yield relatively low, whereby it decreases from one filter assembly to the other. French Patent Specification No. 2,153,334 discloses a filter assembly which has disc-shaped filter elements which are connected to a rotatable hollow shaft. The sludge arrives from above on the filter assembly, whereby the filtrate is collected in the central hollow shaft, whilst the concentrated sludge is again discharged from an outlet at the bottom. This apparatus is however of cumbersome structure, especially because of the problems arising with the sealing of the rotating hollow shaft.